Our previous work on physiological and pharmacological properties of extraocular muscles has demonstrated the presence of a functionally important population of tonic or multiply innervated fibers in rat inferior rectus muscle in addition to the twitch or singly innervated fibers, found in all other mammalian muscles. These two types of muscle fibers have been further identified with electrophysiological techniques. The experiments to be carried out are designed to: a) continue the characterization of the electric and synaptic activities of the fibers and to correlate these properties to their morphological features and b) study the effect of some cholinergic drugs on these muscles. The fibers will be impaled with microelectrodes filled with 3 M KC1 and a dye solution (Fast green, Procion dyes, etc). The type of spontaneous and evoked synoptic activities and the presence or absence of action potentials will be determined and used to classify the fibers as singly or multiply innervated. The fiber will be then marked by depositing the dye iontophoretically. The muscle will be then fixed, the labelled fiber removed and processed for LM and EM for morphological classification. The drugs that will be tested are: acetylcholine, succinylcholine and carbamylcholine. These are assumed to act at the level of the motor end-plate and therefore to stimulate mainly the multiply innervated fibers. They could provide a way of studying selectively the contribution of these fibers to the tension output of these muscles. Besides, the possibility that an adrenergic mechanism is involved at least in the effects of succinylcholine--as has been postulated for cat "in vivo"--will be studied. For this purpose the effects of epinephrine and propranolol as well as other adrenergic blocker agents will be studied.